A field and laboratory method for monitoring the concentration and isotopic composition of soil CO2

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Abstract
The stable isotope composition of nmol size gas samples can be determined accurately and precisely using continuous flow isotope ratio mass spectrometry (IRMS). We have developed a technique that exploits this capability in order to measure δ13C and δ18O values and, simultaneously, the concentration of CO2 in sub‐mL volume soil air samples. A sampling strategy designed for monitoring CO2 profiles at particular locations of interest is also described. This combined field and laboratory technique provides several advantages over those previously reported: (1) the small sample size required allows soil air to be sampled at a high spatial resolution, (2) the field setup minimizes sampling times and does not require powered equipment, (3) the analytical method avoids the introduction of air (including O2) into the mass spectrometer thereby extending filament life, and (4) pCO2, δ13C and δ18O are determined simultaneously. The reproducibility of measurements of CO2 in synthetic tank air using this technique is: ±0.08‰ (δ13C), ±0.10‰ (δ18O), and ±0.7% (pCO2) at 5550 ppm. The reproducibility for CO2 in soil air is estimated as: ±0.06‰ (δ13C), ±0.06‰ (δ18O), and ±1.6% (pCO2). Monitoring soil CO2 using this technique is applicable to studies concerning soil respiration and ecosystem gas exchange, the effect of elevated atmospheric CO2 (e.g. free air carbon dioxide enrichment) on soil processes, soil water budgets including partitioning evaporation from transpiration, pedogenesis and weathering, diffuse solid‐earth degassing, and the calibration of speleothem and pedogenic carbonate δ13C values as paleoenvironmental proxies. Copyright © 2008 John Wiley & Sons, Ltd.